The Tycho-Gaia astrometric solution. How to get 2.5 million parallaxes with less than one year of Gaia data

@article{Michalik2014TheTA,
  title={The Tycho-Gaia astrometric solution. How to get 2.5 million parallaxes with less than one year of Gaia data},
  author={Daniel Michalik and Lennart Lindegren and David Hobbs},
  journal={arXiv: Instrumentation and Methods for Astrophysics},
  year={2014}
}
Context. The first release of astrometric data from Gaia will contain the mean stellar positions and magnitudes from the first year of observations, and proper motions from the combination of Gaia data with Hipparcos prior information (HTPM). Aims. We study the potential of using the positions from the Tycho-2 Catalogue as additional information for a joint solution with early Gaia data. We call this the Tycho-Gaia astrometric solution (TGAS). Methods. We adapt Gaia's Astrometric Global… 

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